Method and cutting insert for cutting screw threads in metal work pieces

ABSTRACT

A thread-cutting insert includes a body and a cutting tip projecting therefrom. The cutting tip includes: a cutting edge formed by a nose edge at a front end of the cutting tip, an active cutting edge disposed on one side of a bisector of the cutting tip, and a passive edge disposed on the other side of the bisector. One thread flank is cut in a workpiece by the active cutting edge. The other thread flank is cut by the nose edge. The passive cutting edge extends rearwardly from the nose edge and is recessed inwardly from the thread flank being cut by the nose edge.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a cutting insert forthreading. The method aims to produce a thread in a rotary work piece,wherein a threading insert, including at least one cutting tip, createsthe shape of the thread through a number of passes along the work piece.The threading insert aims to create threads including two thread flanksin a work piece, by flank infeed and comprises at least one cutting tipprovided with cutting edges.

PRIOR ART

In known indexable threading inserts the cutting tip is in top viewidentical with the profile of the finished the thread, which means thatat flank infeed, wear develops on the passive flank of the cutting tipdue to abrasion from the work piece. Thereby these cutting inserts cannot attain optimal tool life or reduce the time for processing atthreading. At flank infeed a chip is obtained with rectangular crosssection, which is easy to shape and handle in comparison with a chipfrom radial infeed, when the chip assumes a rigid V-shaped crosssection. This fact together with a more efficient heat deduction fromthe cutting tip makes it possible to work with bigger chip thickness andto complete the thread with the same number of passes as by radialinfeed. When conventional threading inserts are fed inwardly, parallelwith one flank of the thread, the cutting edge facing said flank willnot perform any cutting work but will only drag along said flank;something which can have an abrasive effect on the cutting edge andwhich can give bad surface finish on the thread flank. One has tried tofeed the cutting tip at an angle which is smaller than the flank angleto avoid these negative effects, but then chips obtained from the sideof the minor cutting edge have been thin and entangled, which hasinfluenced tool life and surface finish.

OBJECTS OF THE INVENTION

One object of the present invention is to provide a cutting insert forflank infeed, which has good tool life.

Another object of the present invention is to provide a cutting insertfor flank infeed, which reduces the time for processing at threading.

Another object of the present invention is to provide a cutting insertfor flank infeed, which requires less energy to cut.

Still another object of the present invention is to provide a method forflank infeed, which gives good production economy.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a threading insert according to the present invention in atop view.

FIG. 2 shows the threading insert, in a side view.

FIG. 3 shows the tip of the threading insert in engagement with a dashedwork piece. FIG. 3 is similar to FIG. 3 but references additionalcharacteristics of the insert tip.

FIG. 4 shows a threading period of five passes with the tip of thethreading insert.

FIG. 5 shows the tip of the threading insert in engagement with a workpiece.

FIGS. 6 and 7 show alternative embodiments of cutting corners onthreading inserts according to the present invention.

FIGS. 8 and 9 show alternative embodiments of threading inserts fortrapezoidal threads, according to the present invention.

FIGS. 10 and 11 show cross sections of the cutting inserts in FIGS. 8and 9, respectively.

FIG. 12 shows the tip of the threading insert according to FIG. 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIGS. 1 and 2 a cutting insert according to thepresent invention is designated by 10. The cutting insert preferably ismade by coated cemented carbide. The cutting insert has a substantiallytriangular basic shape and comprises an upper and a lower side 11 and 12respectively. The basic shape can also be square for example, such as isshown in FIG. 9. The sides 11 and 12 connect substantiallyperpendicularly to side surfaces 13, 14 and 15. The side surfaces forman acute angle of about 60° with each other. The cutting insert has acentral hole 16 for receiving a fastening device for clamping of thecutting insert against a cutting insert pocket of a holder, not shown.The cutting insert is provided with three substantially identicalcutting corners 17 in the areas for the imaginary intersection of theside surfaces. The cutting insert corner 17 comprises a cutting tip 18,a cutting edge which runs along the intersection between a chip surface19 and a clearance surface 20, a topping surface 21 and a broken chipbreaker rim 22. The cutting insert has a center axis CL and each cuttingtip 18 has a bisector B. The bisector is substantially parallel with theassociated edge surface 15 and extends on the side of the center axis ata distance L. The distance is 10 to 30% of the imaginary edge length ofthe cutting insert.

In FIG. 3 an enlarged cutting corner is shown, which engages a workpiece, the thread 23 of which is dashed. The cutting edge consists of anumber of segments and includes an active major cutting edge 24, a noseedge 25, an active minor cutting edge 24A, a passive edge 26 and atopping edge 27. The cutting tip 18 of the cutting corner projects adistance L₁, measured from a line touching the topping surface 27 to theextreme tip of the nose edge 25. The major cutting edge is provided onone side of the bisector and the minor cutting edge on the other side ofthe bisector while the nose edge cutting intersects the bisector. Themajor cutting edge 24 forms an acute angle α, of about 25 to 30°, withthe bisector B. The major cutting edge softly connects to the nose edge25, which substantially is defined by a radius r, of about 0.05 to 1.5mm. The nose edge further softly connects preferably to the active minorcutting edge 24A or directly to the passive cutting edge 26. The minorcutting edge connects to the passive cutting edge at a the perpendiculardistance L₂ from a tangent T of the nose edge in the extreme points ofthe nose edge. The distance L₂ is bigger than or the same as the maximumdepth of infeed per pass, but less than the half of the distance L₁ andpreferably less than 2.5 mm and is preferably of the size of the radiusr. Also the minor cutting edge or the part of the nose edge which isclosest to the passive cutting edge forms the angle α with the bisector.The passive cutting edge 26 forms an angle β with the bisector, which issmaller than the angle α by an angle δ. The angle δ is 4 to 8°,preferably about 5°, less than the angle α. The passive cutting edgeconnects to the topping edge 27 via a concave substantially passive,curved edge 28, defined by a radius R, the size of which depends on theactual thread profile but the radius r is bigger than the radius of thenose edge. The cutting tip thus has a passive edge 26, which at leastpartly is provided on one side of an imaginary line S extending parallelwith the direction of infeed F for the cutting insert and touching thecutting tip 18, said side facing away from the connected thread flank23. The line S touches the nose edge 25 as well as intersects the curvedcutting edge 28.

From the foregoing, it will be appreciated that the nose edge 25constitutes a first portion of the cutting edge. A section of that firstportion 25 is disposed to the right side of the bisector in FIG. 3 andis adapted to cut one thread flank 23A. The active cutting edge 24constitutes a second portion of the cutting edge disposed on the leftside of the bisector and is adapted to cut another thread flank. Thepassive edge 26 constitutes a third portion of the cutting edge andextends rearwardly from a rear end of the first portion 25 recessedinwardly with respect to the imaginary reference line S.

As shown in FIG. 3A, first line V1 extending perpendicular to thebisector B and passing through an intersection between the first andthird portions 25 and 26 divides the cutting tip into forward andrearward portions. A second line V2 oriented perpendicular to thebisector B within the forward portion intersects the cutting edge at twolocations spaced by equal distances D1 from the bisector B. A third lineV3 oriented perpendicular to the bisector in the rearward portionintersects the cutting edge at two locations spaced by unequal distancesD2, D3 from the bisector B. A shorter one D3 of the unequal distancesextends from the bisector B to the third portion 25 of the cutting edge.

The function of the cutting insert shall be explained closer hereinafterin connection with FIGS. 4 and 5, which show flank infeed withoutmodification by additional tilting. In the first pass I, substantiallythe entire nose edge 25 of the cutting tip is brought, radially into thework piece a predetermined depth (the depth of the infeed), whichseparates from radial infeed only through the axial placement. The depthof the infeed is smaller than or the same as said distance L₂ to avoidpoor surface quality on the passive thread flank. Before pass II iscommenced, the cutting insert is moved in the direction of the arrow Fobliquely inwardly toward the rotational axis of the work piece,parallel to the thread flank 23A. The passive cutting edge 26 therebybecomes cleared from the thread flank in the direction of infeed suchthat abrasion is avoided. Same will apply for the passes III to V. Atthe last pass V the thread also will be topped. The flank infeed is thusperformed in the direction F, obliquely relative to the rotational axisof the work piece and parallel with one thread flank of the thread and apart 26 of the cutting tip is cleared from the thread flank in thedirection F of the flank infeed.

This cutting insert allows considerably larger depth of infeed per passsince unnecessary friction is avoided. This means in turn that thenumber of passes can be essentially reduced; for example from theconventional 8 passes to 5 passes at cutting a thread M12. Therebyreduced time for processing is attained and more details canconsequently be made per shift.

With reference to FIGS. 6 and 7 cutting inserts are shown, the passiveedge 26' respective 26" of which is provided with a clearance by arecess 30' and a skew chamfer 30", respectively, to form a step 40' r40". The recess preferably extends from the minor cutting edge 24'A atthe distance L₂ to the radiussed bridge at the connected topping edge.The recess preferably extends from the passive cutting edge to the lowerside 12 of the cutting insert. The passive cutting edge may be parallelwith the generated thread flank or may form an angle with said flank,such as in FIG. 5. The chamfer 30" preferably extends from the minorcutting edge 24"A at the distance L₂ to the radiussed bridge atconnected topping edge. The chamfer form an obtuse inner angle with theconnected clearance surface 20, said angle preferably being larger than160°. The chamfer provides clearance for the passive cutting edge whenthe cutting insert is mounted leaning in the cutting insert pocket. Bothpassive cutting edges 26' and 26" form an angle with the bisector B,which is smaller than the above mentioned angle α, which is defined bythe relation between the bisector and the major cutting edge and theminor cutting edge, respectively.

In FIGS. 6 and 7, an intersection between the nose edge 25' or 25" withthe passive cutting edge 26' or 26" forms a step 40' or 40" projectingtoward the bisector.

FIGS. 8 to 12 show threading insert for cutting trapezoidal threads,wherein the cutting tip comprises a cutting edge. The cutting edgeconsists of a number of segments and includes an active major cuttingedge 124, a nose edge 125, a minor cutting edge 124A, a passive edge 126and a topping edge 127. The major cutting edge and the minor cuttingedge each form an angle of about 15° with the bisector B. The nose edgeis preferably perpendicular to the bisector. The passive cutting edgemay be parallel with the bisector or be angled relative to the bisector,such as in FIG. 3, or recessed such as in FIG. 6; the most important inthis connection is that the passive cutting edge is provided inside theline S such that the cutting edge does not engage with the connectedthread flank. The passive cutting edge 126 thus can be provided withclearance by means of a recess, said recess extending from the minorcutting edge 1 24A to a radiussed bridge at the connected topping edgeand the recess then extends preferably from the passive cutting edge tothe lower side of the cutting insert. Alternatively the passive cuttingedge can be provided with clearance by a chamfer, said chamfer extendingfrom the minor cutting edge 124A to a radiussed bridge at the connectedtopping edge, wherein the chamfer forms an obtuse inner angle with theconnected clearance surface.

For all above-mentioned cutting inserts the major cutting edge issubstantially uniform with one, connected, generated thread flank andthe passive cutting edge is substantially non-uniform with the other,connected, generated thread flank as well as the distance L₂ is biggerthan or the same as the maximum depth of the infeed for the cutting tipper pass.

The present invention consequently relates to a threading insert whereinfor example a clearance angle of about 5° is provided in the tool'sminor cutting edge side, whereas full profile is provided only on themajor cutting edge side and substantially on outermost point of the tip.Advantages following such a configuration are that lower cutting forces(about 40% lower) arise in comparison with a conventional cutting insertwith a symmetrical cutting tip and that the number of passes can bereduced from for example 8 down to 5 passes for an ISO-profile 2.0 mm.That gives shorter threading periods and more details per cutting edge.The chip surfaces of the cutting inserts may be provided with recessesand/or projections in order to improve chip forming and in certain casesimprove heat deduction.

I claim:
 1. A threading insert adapted to form a thread in a workpiece,comprising a body; a cutting tip projecting forwardly from the body andhaving a cutting edge; a bisector of the cutting tip intersecting afront end of the cutting tip; a first portion of the cutting edgedisposed to a first side of the bisector and adapted to cut one threadflank in the workpiece; a second portion of the cutting edge disposed toa second side of the bisector and adapted to cut another thread flank inthe workpiece; the second portion forming a first acute angle with thebisector; a third portion of the cutting edge extending rearwardly froman intersection with a rear end of the first portion and being recessedinwardly toward the second portion with respect to a imaginary referenceline which extends from the rear end of the first portion and forms asecond acute angle with the bisector that is equal to the first acuteangle, a first line extending perpendicular to the bisector and passingthrough the intersection between the first and third portions dividingthe cutting tip into forward and rearward portions, whereby a secondline oriented perpendicular to the bisector in the forward portionintersects the cutting edge at two locations spaced by equal distancesfrom the bisector, and a third line oriented perpendicular to thebisector in the rearward portion intersects the cutting edge at twolocations spaced by unequal distances from the bisector, with a shorterof the unequal distances extending from the bisector to the thirdportion of the cutting edge.
 2. The threading insert according to claim1 wherein the acute angle constitutes a first acute angle; the thirdportion forming a second acute angle with the bisector which is smallerthan the first acute angle.
 3. The threading insert according to claim 2wherein the second acute angle is 4° to 8° smaller than the first acuteangle.
 4. The threading insert according to claim 2 wherein the secondacute angle is 5° smaller than the first acute angle.
 5. The threadinginsert according to claim 1 wherein the first portion defines the frontend of the cutting tip and is defined by a radius to assume a convexshape; a center of the radius lying on the bisector at a locationbetween the first and second portions of the cutting edge; a forward endof the third portion being spaced by a first distance from the front endof the cutting tip in a direction parallel to the bisector; the firstdistance being less than one half of the total distance of the cuttingtip.
 6. The threading insert according to claim 1 wherein the firstportion defines the front end of the cutting tip and is defined by aradius to assume a convex shape; a center of the radius lying on thebisector at a location between the first and second portions of thecutting edge; the second portion defining an active cutting edge whichsmoothly connects to the first portion.
 7. The threading insertaccording to claim 6 wherein the cutting edge includes a topping edgeportion connected to a rear end of the third portion by a concave edge.8. The threading insert according to claim 1 wherein the acute angle isfrom about 25° to 30°; the first portion being of convex shape anddefined by a radius of from about 0.05 to 1.5 mm; a center of the radiuslying on the bisector at a location between the first and secondportions of the cutting edge; a distance from the front end of thecutting tip to a rear end of the first portion in a direction parallelto the bisector being less than 2.5 mm.
 9. The threading insertaccording to claim 8 wherein the distance is substantially the same asthe radius.
 10. The threading insert according to claim 7 wherein theradius of the first portion is smaller than a radius of the curved edge.11. The threading insert according to claim 1 wherein the third portionis defined by a chamfer of the cutting tip.
 12. A threading insertadapted to form a thread in a workpiece, comprising a body; a cuttingtip projecting forwardly from the body and having a cutting edge; abisector of the cutting tip intersecting a front end of the cutting tip;a first portion of the cutting edge disposed to a first side of thebisector and adapted to cut one thread flank in the workpiece; a secondportion of the cutting edge disposed to a second side of the bisectoradapted to cut another thread flank in the workpiece; the second portionforming a first acute angle with the bisector; a third portion of thecutting edge extending rearwardly from a rear end of the first portionand being recessed inwardly toward the second portion with respect to animaginary reference line which extends from the rear end of the firstportion and forms a second acute angle with the bisector that is equalto the first acute angle, wherein an intersection between the first andthird portions forms a step extending toward the bisector.